A SOFC comprises an oxygen-ion conducting electrolyte, a cathode at which oxygen is reduced and an anode at which hydrogen is oxidised. The overall reaction in a SOFC is that hydrogen and oxygen electrochemically react to produce electricity, heat and water.
The anode also comprises a high catalytic activity for the steam reforming of hydrocarbons into hydrogen, carbon dioxide and carbon monoxide. Steam reforming can be described by the reaction of a fuel such as natural gas with steam and the reactions which take place can be represented by the following equations:CH4+H2O→CO+3H2 CH4+CO2→2CO+2H2 CO+H2O→CO2+H2 
The fuel gas supplied to the fuel cell contains in most cases steam, thus enabling the steam reforming process to occur according to the above equations at the anode surface. The hydrogen produced then reacts in above electrochemical reaction. The steam reforming reaction is, however, very endothermic and a large heat input is therefore required.
A typical temperature distribution in a fuel cell stack with a hydrocarbon feedstock therefore shows a dramatic temperature drop near the inlet of the fuel cell due to the fast endothermic reforming reaction resulting in severe temperature gradients within the cell.
The SOFC is a ceramic composite of three different materials. Ceramic SOFCs have low mechanical strength and in particular low tensile strength. The tensile strength within a SOFC is closely connected to temperature gradients and it is therefore highly important to minimise the temperature gradients and thereby the tensile strength of the SOFC. When the tensile strength in the fuel cell exceeds a given threshold value the cell will crack and the fuel cell will malfunction.
It is to some extent possible to control the tensile strength to an acceptable level by using a hydrogen feedstock, but in the future it is foreseen that natural gas and other hydrocarbon feedstock will become dominant. This will increase the problem dramatically as the endothermic reforming of hydrocarbons will reduce the temperature of the fuel cell in the fuel-inlet area significantly, thereby increasing the temperature gradients and the tensile strength in the fuel cell to an unacceptable level.
Several methods of reducing the temperature gradients are known. Most of these methods involve changes in operation parameters of the fuel cell system such as enhanced airflow to the cathode. Such changes are often connected to increased operation cost of the fuel cell system.